1. Field of the Invention
The present invention relates to a magneto-optical disc which is a mass storage information recording medium.
2. Related Background Art
The magneto-optical disc is a mass storage information recording medium for recording or reproducing information in such a process that a magneto-optical film having the magnetic Kerr effect is formed on a transparent substrate, a linearly polarized laser beam is irradiated onto the film through the substrate, and a fine angle of Kerr rotation caused by the magneto-optical film is read out. The disc of this type has a distribution of birefringence along the surface of its substrate. In using the disc, the birefringence causes a reflected laser beam to have a phase difference between an ordinary ray component and an extraordinary ray component thereof so as to become elliptically polarized light. It decreases the amplitude of magneto-optical signals, raising a problem of lowering the signal level. It is thus considered that the transparent substrate should preferably have birefringence as close to zero as possible.
Generally employed as materials for the transparent substrate are glass, epoxy resins, acrylic resins, and polycarbonate resins. Currently, the most generally used method for forming the substrate is the injection molding method, which is the highest in productivity. Popularly used among the resins, which are the polycarbonate resins stablest in heat resistance, water absorption property and other mechanical properties. The polycarbonate resins are materials which are normally considered to be liable to cause birefringence because of the orientation arising from the molecular structure thereof. However, various technical improvements achieved heretofore reduced the birefringence of polycarbonate substrates down to a level of nearly zero. For example, an improved polycarbonate resin has flowability several times higher than the polycarbonate resins of general-purpose grade, which greatly reduced the birefringence. On the other hand, the injection molding machine has been improved to develop a disc molding machine with high-speed and high-responsibility injection capability, enabling stable molding. Also, a disc mold or die is improved so as to permit uniform control of die temperature and to obtain suitable gate cross section and disc gate cutting, so that the birefringence can be made nearly zero even with the substrate being made of the polycarbonate resin.
The inventors of this application have made magneto-optical disc substrates of a polycarbonate resin low in birefringence utilizing the above conventional technology and investigated signal properties for discs made by forming a magneto-optical recording film on each of the substrates. The investigation clarified that the discs of the polycarbonate substrates were slightly lower in amplitude of magneto-optical signals than glass discs.
The trend discussed above becomes more enhanced as the rotational frequency of the disc increases, which heretofore rendered the polycarbonate resins unsuitable for use in high speed rotation. The investigation by the inventors of this application found the following cause of such a reduction in signal level during rotation at high speed. The centrifugal force caused by the high speed rotation produces the birefringence inside the substrate, which acts in a direction to adversely affect the plane of polarization of linearly polarized light used in an optical system in an employed drive.